Homotherium was a sabercat that survived until the last Ice Age. This skull is from the Muséum national d'Histoire naturelle in Paris.

Many sabertooths have stalked this world. The first sabertoothed mammals appeared over 50 million years ago. The last sabercats, such as Smilodon and Homotherium, went extinct only 10.000 years ago. All in all, five different lineages of carnivorous mammals evolved sabertooth dentition: the ancient creodonts, marsupials and three different lineages of true cats and cat-like carnivores. These creatures were unrelated and lived millions of years apart, yet somehow all evolved canines that were similarly massive and grotesque.

This independent and repeated evolution of saberteeth in different mammalian carnivores seems, at first glance, a testament to the power of natural selection. We assume canines long and slender have evolved to subdue and kill, to slash veins and pierce flesh, to bring down the most powerful of prey. Larger canines equal more carnage, or so our intuition tells us.

But Marcela Randau isn’t so sure. In her latest paper, she and her colleagues propose that saberteeth did not evolve because they were deadly, but because they were sexy.

That’s not as far-fetched an idea as it might first appear. Many modern animals bear exaggerated canines and tusks as sexual ornaments, like walruses, baboons, narwhals and several species of deer (yes, quite a scary sight). It are mostly the males that develop such impressive dental weaponry and use them in competitions with other males, in displays of social dominance or in actual fights. The victors usually secure the right to mate with a female or even entire harem. Since the males with the largest and most elaborate tusks win access to more females, evolution can drive the runaway development of longer and longer canines.

Perhaps this is how sabertoothed carnivores evolved their teeth. Modern cats, like lions, raise their lips and expose their canines when they feel threatened or challenged. Cats are visual creatures that pay much attention to the face and expressions of others. Through their canines, they advertise their strength and aggressive intent: back away, because I have the teeth to back up my threats. Could it be that sabertooths simply took such toothed communication to the extreme? A sabertooth snarl would certainly have sent a clear message to male competitors.

Lions communicate aggressive intent by snarling and growling.

But just because this scenario sounds plausible or imaginable, doesn’t mean it’s true. Comparisons with modern animals that bear their tusks in sexual display can be misleading, especially since none of them are carnivorous hunters, like sabertooths were. It’s evident that a deer won’t use its canines for killing (one hopes), but the same can’t be said for a sabertooth. How then does one investigate whether saberteeth evolved for the hunt or for sexual conflict?

Randau’s argument is based on proportions. As an animal matures and grows, most of its body parts will grow with it in a steady, lockstep fashion. But the rules are different for sexually selected ornaments and weapons. In this case, bigger means much better. The larger a buck can grow its antlers, or the more elaborate the peacock tail becomes, the more likely it is to find a mate. Males therefore tend to devote a disproportionate amount of energy in the development of sexual features, with the largest males displaying the most eye-catching tails, frills and feathers. Such patterns of lopsided growth, what biologists call ‘allometrical growth‘, can therefore be a sign of sexual selection at work.

To see whether sabertooth canines display allometrical growth, Randau and her colleagues investigated how canine size of sabertoothed carnivores scaled with the length of their skulls. If larger skulled sabertooths have much larger canines than smaller animals, this could mean they were partly shaped by sexual selection. The research team made the same comparison for extinct and modern cats without sabertooth dentition. And as a sanity check, Randau made the same calculations for the shearing teeth scale, which are used for slicing meat and lie just behind the canines. Since they aren’t visible, they’re unlikely to have a role in display and should therefore grow at the same rate as the skull and rest of the body.

Randau found that both sabertoothed carnivores and non-sabertoothed cats develop disproportionally large canines as they grow, but it were the sabertooth canines that were the most extreme. The proportion of the shearing teeth, did not change for skulls of differing lengths, indicating they evolved under a regime of natural selection. Randau’s team concluded that saberteeth were honed, at least in part, by sexual selection.

Of course that doesn’t mean that saberteeth had no role in killing or wounding prey. A sharp and slender tooth can still be a potent weapon. Longer teeth bite deeper and inflict more damage, at the risk of breaking as the prey struggles. However, body parts that are ‘merely’ functional don’t grow progressively larger in larger animals. According to Randau, sexual selection can explain why saberteeth grew as large as they did, whereas natural selection constrained how large they could become while still remaining (somewhat) practical.

Julie Meachen, a sabertooth researcher who was not involved with the study, thinks Randau’s hypothesis is plausible. Meachen herself has investigated the differences between the prey killing arsenal of two different types of sabertoothed carnivorns: dirk-tooths with canines long and slender, and scimitar-tooths, which had shorter and more serrated canines. Meachen found that the sabertooth with larger canines, are also the ones that have the strongest fore-limbs. In her paper makes the case that since slender canines are more prone to break, these creatures needed robust limbs to grapple and hold down prey as they made the kill.

“Originally, it was a chicken or egg problem which came first: the long canines or the robust forelimbs”, says Meachen. “But I see the ideas as being compatible in this way: sexual selection drove the canine length to get longer, but in order for the long canines to remain functional, they also needed their forelimbs to be robust and strong.” Fangs before paws, in other words.

Skulls can’t talk, so answers to the question how sabertoothed carnivores hunted and bred will never be definitive. It’s also clear that a single answer will never suffice for a group as large and varied as sabertooths. But the patterns that sabertooth researchers like Randau and Meachen have uncovered are real. And if their interpretations are correct, sabertooths were every bit as menacing as sexy.

15 Comments

Usually, extreme sexual ornamentation is a male province, which this article acknowledges (“It are mostly the males that develop such impressive dental weaponry and use them in competitions with other males, in displays of social dominance or in actual fights.”).
Thus, it would seem necessary to discuss whether sabertoothed cats show signs of any great sexual dimorphism in the saber department. Do the females lack saberteeth, or are their saberteeth much smaller than those of males of the same overall size?
I’m disappointed that the article fails to discuss the issue; it’s central to any discussion of sexual ornamentation.

Thanks for your comment! To my knowledge, sexual dimorphism in sabertooths has only been investigated for Smilodon, by Van Valkenburgh and Sacco (Sexual dimorphism, social behavior, and intrasexual competition in large Pleistocene carnivorans). The researchers found no evidence for dimorphism in this species. This doesn’t strengthen the case for sexual selection, but doesn’t necessarily weaken it either. In their paper, Randau and her colleagues mention certain sexual traits are ‘mutually selected’, like ultraviolet plumage in blue tits.

Lucas, thanks for responding.
I have the impression that most if not all species with mutually selected sexual ornamentation are monogamous or mostly monogamous. (They’re also mostly birds, but I suppose that could be an artifact of the higher frequency of monogamy in birds than in other vertebrates.)
Sabertooth cats could have been monogamous, I guess. Current cats are not, but none of the various sabertooths were all that closely related to current cats.
BUT monogamy in mammals is very rare. (Gibbons come to mind. And humans, but our sexual ornaments are not mutually selected.) A monogamy hypothesis for sabertooths would need other evidence besides the fact that sabertooths of both sexes have saberteeth. And except for monogamy, I don’t know of any other reason for mutually selected sexual traits.
So:
1. Is there any reason other than their teeth to suspect monogamy in all the various sorts of sabertooths; or
2. Are there known reasons other than monogamy that lead to mutually selected sexual traits?
Unless one or both of those questions has a Yes answer, I think it’s a lot likelier that saberteeth were more functional than ornamental.

Unfortunately the paper seems not to compare canines of young and sexually mature individuals, but small and large species. Which puts the whole theory on it’s head.

Strange, because cats like lion and Smilodon are well known, and it could be possible to assemble time series of growing cats.

I think more plausible is that larger sabertooth species attacked larger prey, which required progressively longer canines to cut through the skin and fat layer to the vital organs. Which is basically true for modern largest animals, which have disproportionately tough skin and fat layer.

So, allometry of sabertooth canines would be a by product of allometry of herbivore bodies. Larger herbivores like bison and elephants are allometrically more bulky compared to small ones like gazelles and deer.

@timcliffe #3 – Don’t forget that evolutionary adaptations in general are more easy to acquire if they are *not* single sex only, as for sexual dimorphism there must be an extra activator or inhibitor (which takes extra mutations).

@jerzy #4 – Currentday lions are very well equiped to take down elephants. You don’t need large sabre teeth for that. So I think that what you think is more plausible isn’t all that plausible.

@timcliffe: The social and breeding system of sabertooths like Smilodon has been hotly debated in recent years. Given the high number of Smilodon remains found in places like the La Brea tar pits, some researchers think Smilodon lived in large (family?) groups (see here for example, and here for discussion). But even assuming that Smilodon lived in groups, this does not resolve the question whether they were monogamous (like canids) or polygamous (like lions).

@Jerzy: The ‘megaherbivore hypothesis’ has recently been tested by a Andersson et. al. The researchers found that the depth of the killing bite decreased dramatically for prey of increasing size and concluded that sabertooths likely hunted prey of a very similar size as modern carnivores (so very few elephants, sorry killianh). It appeared that mostly that the biting mode of sabertooths differed, not their choice of prey.

Lucas, I went back and read the three abstracts you referred to and I see that Van Valkenburgh and Sacco find a lack of sexual dimorphism in Smilodon canines, and that they infer some possibility of monogamy from that lack. I was also very impressed by Carbone et al.’s analysis that Smilodon may well have been social, based on their frequency at La Brea compared to (social) lions’ frequency in experiments using “dying herbivore playbacks.”

Taking the two things in combination (little sexual dimorphism; lion-like frequency at La Brea), I have to say the “mutually selected” sexual ornament hypothesis seems more plausible than at first glance. It’s hard to picture a cat with a wolf-like social structure, but the facts seem consistent with the idea. I hope we’ll see more on this in the future.

Kilianh, your point is well-taken as regards the initial acquisition of an adaptation (“. . . evolutionary adaptations in general are more easy to acquire if they are *not* single sex only, as for sexual dimorphism there must be an extra activator or inhibitor (which takes extra mutations).”). However, sexual ornaments are almost by definition costly, and natural selection will take advantage of any of the extra mutations you refer to reduce that unnecessary cost in the non-competing sex (usually females). We know this because of the fact that sexual dimorphism is the rule in species where ornamentation of some kind is used in sexual contests. “Mutually selected” ornamentation is unusual, and as far as I can tell is limited to monogamous species.

If Smilodon lived in groups, what sort of groups were they?
I previously referred to Carbone et al.’s inference of a possibly wolf-like social structure for Smilodon (a wolf pack consist of a monogamous alpha pair plus various non-breeding descendants, and they don’t show much if any sexual dimorphism in their canines).
I should have added that a lion-like social structure (a pride consists of one or two males that mate with all the females) seems Unlikely, because lions have lots of sexual dimorphism, whereas Smilodon apparently did not.

I read this paper when it came out and was rather disappointed. Some problems that I see are:

1) The authors looked at interspecific, rather than infraspecific allometry (as has already been pointed out above). Most of the work on allometry in sexually selected traits has been done on intraspecific allometry.

2) No correction for phylogeny, so no means of knowing how much of this pattern is due to convergent evolution and how much is a consequence of different species sharing common ancestors that also had (for example) big canines.

3) Positive allometry does not always mean sexual selection, and some sexually selected traits are isometric or negatively allometric.

In my opinion this makes it rather hard to draw much inference from these data.

Contra RJKnell, multiple-species comparisons have been used a great many times in studies of sexual traits. Malte Andersson’s comprehensive book Sexual Selection includes discussion of many such studies.
In fact, one of the studies Andersson cites is strikingly similar to the topic we’re discussing here:
“Sexual dimorphism in body and relative canine size in primates with different mating systems” (Clutton-Brock and Harvey 1984).
This study of saberteeth cannot be dismissed because it compares different species. That’s how you can tell that some species (or groups of species) have canines that seem out of proportion to their overall size.

All of these were cited in relation to the passages that initially caught my eye:

“In primates, perhaps the most thoroughly studied group, male relative to female body and canine sizes are larger in polygynous than monogamous species. Moreover, they are larger in strongly polygynous forms . . . than in species with multimale troops . . . “ (Andersson p. 86).
“The issue is complicated further by a general tendency for sexual size dimorphism to increase with body size” (Andersson pp. 87-88).

I regret misstating the name of Clutton-Brock and Harvey 1984. But the question raised in “The Sexy Sabercat” concerns sexual selection, and my point in bringing those primate canine studies up remains the case: many studies bearing on sexual selection do involve cross-species comparisons.

For those who are interested: science writer Brian Switek has just reported on a sabertoothed opossum, in which the males have elongated fangs. The males seem to use their teeth mainly in sexual conflict, but also for hunting. Read the full story here.